There have been a few times in my life, where I believe I have experienced the phenomenon called "Runners High", or the euphoric state during a run where you feel like you can keep running for forever.

@Jdelage Clearly you've never experienced this "state of being". It's quite an enjoyable experience once you get it. I't almost an accomplishment in my opinion.
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KronoSMar 11 '11 at 19:49

Fair point but heroin is enjoyable too... And I'm using this example because endorphines are often compared to opiates.
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JDelageMar 11 '11 at 21:19

@Jdelage This experience is a natural thing, and is something that is good overall. There is, to my knowledge, no ill sideeffects.
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KronoSMar 11 '11 at 22:27

@KronoS, a lot of things are natural and/or have no bad side-effects, when done at a natural level. But doing something for hours more than it normally occurs, changes the picture into something possibly harmful. Or as toxicologists say, "the dose makes the poison".
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John CJan 24 '12 at 13:17

8 Answers
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In a groundbreaking 2003 experiment, scientists at the Georgia Institute of Technology found that 50 minutes of hard running on a treadmill or riding a stationary bicycle significantly increased blood levels of endocannabinoid molecules in a group of college students.

The endocannabinoid system was first mapped some years before that, when scientists set out to determine just how cannabis, a k a marijuana, acts upon the body. They found that a widespread group of receptors, clustered in the brain but also found elsewhere in the body, allow the active ingredient in marijuana to bind to the nervous system and set off reactions that reduce pain and anxiety and produce a floaty, free-form sense of well-being. Even more intriguing, the researchers found that with the right stimuli, the body creates its own cannabinoids (the endocannabinoids). These cannabinoids are composed of molecules known as lipids, which are small enough to cross the blood-brain barrier, so cannabinoids found in the blood after exercise could be affecting the brain.

@Jojo Our ancestors in the savannah had a quite curious hunting technique: Run towards the prey. It'll dart away (think Zebra or Gazelle) a couple yards and then sit down. Keep running towards it. Repeat for a couple of hours, until prey is totally exhausted from darting away. Hence, running for long periods of time is what we're made for and thus our body wants to tell us to enjoy it.
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LagerbaerApr 11 '11 at 17:12

In my experience, runner's high is only obtained after you "hit the wall" and get your "second wind". You have to run past the wall and hit a second wall before you get "runner's high". I always thought it was a result of the release of endorphins.

In order to reach a runner's high faster, you have to run at a high level of intensity for an extended period of time. The more you train, the better level of conditioning you reach, which means you have to continue to push yourself even harder to reach a real runner's high.

Evolutionarily, I think it's a survival mechanism. It allows you to push past all feedback from your body that tells you to stop, and continue anyway.

Second Wind, Runner's High, etc... are probably caused by reaching the point where your body begins to burn fat for fuel. Fat is a much richer source of energy and is easier to get useful fuel from than sugar, so when your body, which has been burning sugar, starts burning fat you'll feel like you suddenly have a greater-than-expected amount of energy.

The previous answer dealing with endocannibinoids mentions that this is caused by lipids, which are small enough to make it into the brain from the bloodstream. These lipids are likely the byproduct of your body beginning to burn stored fat for fuel.

When you said "probably caused", do you have any references?
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BaumrMar 4 '14 at 14:29

Research that can show causal relationships between a concept such as "Runners High" and an observable physical phenomenon is pretty sparse. So no, not really. Just observations of my own experience extrapolated to bits and pieces of research that I've read that seems to make sense for me.
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alesplinMar 4 '14 at 19:02

I see. Would be good to see the bits and pieces of research as well.
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BaumrMar 5 '14 at 15:01

2

It's been quite a while. I'll see if I can hunt them down again.
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alesplinMar 5 '14 at 20:53

I ran long-distance track in high school (I'm currently in college) and still run to this day. However, it was during season when I trained a lot that I experienced this sensation - I know it's a release of endorphins which give you the Euphoria, but I also noticed that after running I was in a great mood as well. The endorphins last quite a while.

Also, I could reach the Runners High very quickly when I was in really good shape (generally after my first half of a mile). I think it differs with everybody but I do remember it becoming more common as I ran more often. Hope this helps

I think that the feeling you are referring to is more often called being "in the zone". I just don't think most people reach a runner's high in the beginning of a run. The feeling you described that you feel after a run sounds more like what I experienced and considered to be a runner's high.
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Natalie BarnettApr 7 '11 at 3:32

This is wikipedia's answer, which I've been led to believe are the reason for the sensation both during and after exercise.

Endorphines ... resemble the opiates in their abilities to produce analgesia and a feeling of well-being.

[...]

A publicized effect of endorphin production is the so-called "runner's high", which is said to occur when strenuous exercise takes a person over a threshold that activates endorphin production. Endorphins are released during long, continuous workouts, when the level of intensity is between moderate and high, and breathing is difficult.

For me the only reasonable answer is: to enable to a runner to continue with the activity. Otherwise, he would just stop, feeling exhausted. From evolutionary point of view, there must be a reason for a long run/activity and ones who had ability to force themselves to perform longer, had better chances to survive.
Feeling better was not the aim just for itself, like it is now, in modern times.

The term runners high, refers to the state of feeling "high" after a run... I think what you're asking about is "second wind" there are various theories regarding the second wind.

The first one being that after a certain amount of time running your body reaches a state of equilibrium between oxygen and pyruvic acid ( which turns into lactic acid if it is not oxygenated quickly enough) which results in this pyruvic acid being converted into ATP (adenosine tri-phosphate). Therefore giving you a new source of energy.

The second theory is that of the endorphins mentioned earlier

From what I can gather from extensive reading on the topic, there is no way to train for the second wind, however I experience it after 3-4 minutes over my usual running time.

Do you have any additional reading regarding that first theory?
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BaarnApr 22 '13 at 10:30

A second wind does not occur after 3-4 minutes, more likely you are not warming up properly. Also, lactic acid is a source of energy, and is resynthesized to glucose in the liver (Cori Cycle). Also, in anaerobic glycolosis, lactic acid is used to produce pyruvate which is another source of ATP production. Lactic acid (Contrary to popular belief) is not responsible for muscle burn, it is not just a waste product, and it is also not responsible for delayed (24-48 hour after exercise) soreness. -1.
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JohnPApr 22 '13 at 15:24

It says 3-4 minutes over what I would normally run... Which is between 30 ad 35 minutes
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minimattMay 1 '13 at 9:50